Image-forming devices, such as laser printers, that are well known in the art have been conventionally provided with a toner sensor for detecting the amount of residual toner in a toner cartridge mounted in the image-forming devices.
For example, a monochromatic image-forming device disclosed in Japanese unexamined patent application publication No. HEI-7-306582 includes a toner sensor having a light-emitting unit for emitting light into a toner cartridge and a light-receiving unit for receiving the light passed through the toner cartridge. The image-forming device detects changes in the light received by the light-receiving unit as an agitator agitates toner in the toner cartridge, thereby detecting the remaining amount of toner. In this monochromatic image-forming device, it is also determined whether or not the toner cartridge is being mounted in a main casing based on whether light emitted by the light-emitting unit was blocked by the side surface of the toner cartridge.
Since this image-forming device uses the toner sensor instead of a dedicated sensor for detecting the presence of the toner cartridge, this technology simplifies the assembly of the image forming device and reduces manufacturing costs.
In recent years, color image-forming devices capable of forming color images on paper have become widespread in use. These image-forming devices are provided with four toner cartridges, each filled with toner of a different color. One color image-forming device well known in the art is provided with a drawer section that can be pulled out of a main casing of the image-forming device and that accommodates the four toner cartridges mounted in a linear juxtaposed relationship. This color image-forming device can detect when the four toner cartridges have been pulled out of the main casing by providing a single sensor dedicated to detecting the presence of the toner cartridges or by employing one of the toner sensors as a sensor for detecting the presence of the toner cartridges.
However, with a color image-forming device having this construction, there may be cases in which a toner cartridge positioned nearest the operator in the direction for pulling the toner cartridges out of the main casing runs out of toner and needs to be replaced. In this case, the operator may simply pull out the drawer section a sufficient distance to replace the nearest toner cartridge and subsequently reinsert the drawer section into the main casing. In such a case, the sensor provided to detect the presence of the toner cartridges detects that the four toner cartridges have not been withdrawn from the main casing. Accordingly, a message for prompting a user to replace the toner cartridge remains displayed on a display panel provided to the image forming device, even though the toner cartridge has been replaced.